In the discussion of the background that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art.
Manufacture of cutting inserts by powder metallurgical methods includes compaction in a press of a powder into a body, and subsequent sintering of the body to produce a cutting insert. Compaction takes place under high pressures obtained through large axially opposing forces generated by top and bottom punches moved into a cavity formed in a die containing the powder. The pressed body generally has a shape such that it easily can be removed from the die. This means that the chip breakers generally provided on the rake face of an insert are formed by the top and bottom punches.
Cutting tool inserts generally have a hole for clamping them to a tool holder by a screw. The holes generally have a noncylindrical shape such as trumpet style in order to more securely fasten the inserts to the holder.
Inserts can be either radial or tangential. Radial inserts are oriented in such a manner that the cutting forces are directed along a thinner dimension of the insert. The clamping hole extends from a rake face to an opposite face, i.e., a bottom face or a rake face. The rake faces generally having chip breakers formed by the punches in the pressing operation. Clamping holes are in this case parallel to the main pressing direction and are easily formed with satisfactory accuracy.
Tangential inserts are oriented in an insert holder in such a way that during a cutting operation the cutting forces are directed along a thicker dimension of the insert. An advantage of such an arrangement is that the insert withstands greater cutting forces. In other cases limitations in the available space for mounting may motivate the choice of a tangential insert design. In the case of a tangential insert the clamping hole is perpendicular to the main pressing direction and such inserts have to be produced by more complex methods.
Methods for manufacturing tangential cutting inserts having a noncylindrical cross-hole, FIG. 1, include a method in which the powder is compacted in a die by top and bottom punches to a body. The cross-hole is subsequently machined in the body which then is sintered. The hole obtained in this way fulfills the dimensional requirements. However, the machining of the body is very time consuming which makes the insert expensive to manufacture. It is therefore desirable to produce inserts with a cross-hole directly in the pressing operation.
U.S. Pat. No. 6,645,426 discloses a method comprising a step of filling powder into a cavity formed of a die having in a vertical direction a die hole including a cross-hole. The powder is filled in the cavity and pre-compacted by the top and bottom punches. A punch-out pin is then inserted into the powder, the shape of the cross-hole is punched out by the punch-out pin. Thereafter, the powder is compacted by means of the top and bottom punches to its final density. The punch-out pin is thereafter pulled out of the green compact and the green compact is taken out so that a completed product can be obtained. A major weakness with this method is its limitation to produce only cylindrical cross-holes and the waste of powder due to the punched-out volume. There is also a potential risk of defects around the hole entrances.
U.S. Pat. No. 6,986,866 discloses a method to produce inserts with a cross-hole directly in the pressing operation based on uni-axial pressing in a die by top and bottom punches by:                positioning the bottom punch in the die below a core bore,        positioning movable core rods in the cavity in a position where the rods are in contact with each other,        filling the cavity with powder,        positioning the powder about the core rods to control the location of the opening after sintering,        compressing the powder uniformly about the core rods,        retracting the top and bottom punches for decompression of the green part,        retracting the core rods from the cavity and        ejecting the green part from the die.        
In this manufacturing method it is difficult to obtain a uniform density due to significant differences of the ratio of pressed height to fill height which makes it necessary to modify the shape of the core pins in order to obtain the desired shape and dimensions after sintering. Another drawback is the obvious risk of powder sticking to the end surfaces of the core rods making it impossible to move them into the desired closed position or causing damage of the core rods in the contact area. A third drawback is the flash formed in a direction radial to the core rods where the core rods meet.
JP 10-146695 discloses a method to obtain a green compact by means of uni-axial pressing using two top and/or two bottom punches and a core rod during the compaction of the powder and thereby avoid modifications of the core pin. The problem with this method is to obtain a sufficiently uniform density distribution around the hole since the surface of the hole towards the top punches and bottom punches is curved. The method will also cause flashes in the partings between the divided top and bottom punches.